1. Field of the Invention
The present invention relates to novel thermoplastic elastomers based upon silicones, and, more especially, to elastomers which are block copolymers comprising polysiloxane and urethane segments. The invention also relates to the preparation of such elastomeric copolymers.
2. Description of the Prior Art
Thermoplastic silicone elastomers are well known to this art, for example, those described in French Pat. Nos. 2,300,778, 2,362,183 and 2,414,519 and French Application No. 78/26,245, assigned to the assignee hereof. These elastomers are prepared by a hydrosilylation reaction between a difunctional .alpha.,.omega.-bis-vinylsilyl compound and an .alpha.,.omega.-dihydrogenopolysiloxane. To produce the elastomers, it is therefore necessary to prepare the difunctional .alpha.,.omega.-bis-vinylsilyl compound as an intermediate, the latter being itself prepared from an unsaturated silane and a coupling agent. It is for this reason that the preparation of these thermoplastic organosilicon copolymers can prove relatively expensive, which constitutes a formidable obstacle to the economic development of said elastomers.
Furthermore, numerous block copolymers containing polysiloxane blocks are also known and which contain various other segments, such as polyester, polyether and polyurethane, and the like. Polysiloxane/polyurethane copolymers are thus known, such as those described in French Pat. Nos. 1,370,886 and 1,371,405 and in Russian Pat. No. 262,385. These copolymers provide solid substances having very little or no elastomeric character.
The block copolymers of the prior art are prepared either by reacting an organic polyisocyanate with "macrodiols" comprising polysiloxane segments [French Pat. No. 1,370,886, Russian Pat. No. 262,385, the article in Vysokomol Soedin, series B, 14, 9, pages 682-684 (1972), and the like], or by the condensation of a "short" organic diol with an .alpha.,.omega.-bis-(isocyanatoalkyl)-polysiloxane (French Pat. No. 1,371,405, and the like).
It has now been determined that the various processes of the prior art do not enable the preparation of copolymers comprising polysiloxane and polyurethane functions which possess good elastomeric properties. This finding would appear to be the consequence of the incompatibility observed between the long polysiloxane segments and the urethane segments. In other words, the lengthening of the polysiloxane block required in order to obtain good elastomers resulted in the appearance of heterogeneity in the medium when reacting a macrodiol (or a macrodiisocyanate) containing a long polysiloxane chain and a short polyisocyanate (or short polyol).